Matthew Wilcox [off-list ref] wrote:

So ... a page with both flags cleared should have a refcount of N.
A page with one or both flags set should have a refcount of N+1.
...
How is a poor filesystem supposed to make that true?  Also btrfs has this
problem since it uses private_2 for its own purposes.

It's simpler if it's N+2 for both patches set.  Btw, patch 13 adds that - and
possibly that should be merged into an earlier patch.

David